1. Field of the Invention
The present invention relates to apparatus and methods for inspection of surfaces of samples such as semiconductor devices.
2. Description of the Related Art
In manufacture of semiconductor devices, defects of circuit patterns formed on wafers are detected, for example, by comparison of images. For example, Japanese Patent Application Laid-Open No. 59-192943; J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991); J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992); SPIE Vol. 24, No. 39, pp. 174-183; and Japanese Patent Application Laid-Open No. 05-258703 disclose methods for pattern inspection by pattern comparison according to “SEM process” using a scanning point electron beam.
These techniques are used for detection of defects each having a small size under the resolution limit of optical microscopes, such as minute etch residues or minute pattern defects and for detection of electric defects such as open defects of fine via-holes and contact holes. These techniques must yield pattern images at a very high speed in order to provide a practical inspection speed. To ensure a satisfactory signal to noise ratio (a S/N ratio) of the images obtained at a high speed, these techniques use a beam electric current higher hundred times or more (10 nA or more) that of conventional scanning electron microscope.
Japanese Patent Application Laid-Open No. 7-249393, No. 10-197462, No. 2000-340160 and No. 11-108864 mention apparatus for high-speed inspection by means of a “projection process” in which a rectangular electron beam is applied to a semiconductor wafer, and images of back scattered electrons, secondary electrons or reflected electrons are formed with lenses. The reflected electrons are reflected without impingement on the wafer due to a formed retarding field for primary beam.
Separately, a technique for obtaining an image of the outermost surface of a sample using a “mirror microscope” has been proposed (e.g., Rheinhold Godehardt, ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 94, p. 81-150). In this technique, a potential is applied to the sample to yield an electric field, and the electric field reflects an electron beam in the vicinity of the surface of the sample without impingement on the sample.
However, all the conventional techniques for inspection using electron beams, such as the SEM process and the projection process using backs scattered electrons or secondary electrons, have the following problems.
The SEM process uses an electron beam current higher than that in conventional scanning electron microscopes in order to form images having a satisfactory signal to noise ratio to be inspected. However, the SEM process cannot inspect a sample with a satisfactorily high speed (for a short time), since it two-shapeally scans the surface of the sample with a “point electron beam” formed by converging an electron beam to a point beam.
In addition, the brightness of an electron source used and space charge effect limit increase in the electron beam current. For example, to yield a resolution of about 0.1 μm, the theoretical limit of the electron beam current is about several hundreds nanoamperes, and at most an electron beam current of about hundred nanoamperes is used in practice. A signal to noise ratio of an image is determined by the product of the time to acquire the image and the number of electrons used for the formation of the image, i.e., the magnitude of the electron beam current. To yield a sufficient signal to noise ratio of an image so as to operate image processing procedure successfully, it takes 100 seconds or longer to inspect an area of 1 cm2 on the surface of the sample when a 100-nA electron beam current 0.1 μm in size is used.
In contrast, the projection process can illuminate the sample with a higher electron beam current at once than the SEM process and can yield images at once. Accordingly, the projection process may form images at a much higher speed than the SEM process. However, in the projection process, the secondary electrons are emitted at angles in a broad range and have energy in a broad distribution ranging from about 1 to 10 eV. To form an image of such electrons to thereby form a magnified image of the sample, the great majority of the second electrons must be cut off to yield a sufficient resolution. This can easily be understood from FIG. 6 in LSI Testing Symposium/1999 Proceedings, p. 142. This figure shows the relationship between the negative voltage applied to the sample to accelerate the secondary electrons emitted from the sample and the resolution in image formation of the secondary electrons and shows that the resolution is about 0.2 μm when the voltage applied to the sample is −5 kV.
All the emitted secondary electrons are not always used for the image formation. For example, the calculation in the reference mentioned above uses a beam having an angle of aperture of less than or equal to 1.1 milliradian (mrad) in an image plane after passing through an objective lens. Secondary electrons each having an angle of aperture within this range occupy at most about 10% of the total secondary electrons. In the calculation, the energy distribution of the secondary electrons for use in image formation is assumed at 1 eV. However, secondary electrons actually emitted have a distribution range of energy of equal to or more than several electron volts or more, and some of them have energy of about 50 electron volts. Such secondary electrons having an energy distribution of at most 1 eV constitute only a fraction of secondary electrons having a broad energy distribution as described above.
As thus described, when a high-current electron beam as a sheet beam is applied to form images at once, a sufficient signal to noise ratio of the image cannot significantly be obtained and the inspection time cannot sufficiently be shortened to an expected extent, since the ratio of electrons actually contributing to image formation is low. Likewise, in the projection process using back scattered electrons, electrons are emitted in an amount less by two orders of magnitude than the primary electron beam. Accordingly, the projection process using back scattered electrons cannot yield a high resolution and high speed in inspection concurrently as in the projection process using secondary electrons.
In addition, this technique does not limit the direction of trajectory of the electron beam directed to the sample, illuminates the sample with the electron beam with a broad range of angles and thereby yields insufficient resolution of about submicron order and is insufficient in resolution to inspect current downsized semiconductor devices.